In the past 10 years thousands of sufferers from multiple sclerosis, the progressive disease of the central nervous system, have used decompression chambers in the hope of alleviating symptoms and putting the disease into remission.

But the use of decompression chambers, known as hyperbaric oxygen therapy and most commonly used for divers with the bends, has always been contentious. Sceptics, including the Multiple Sclerosis Society and the majority of neurologists, say that the risks involved outweigh any possible benefits.

New concerns about the safety and regulation of these chambers were voiced last week after a community nurse, who had accompanied a patient into a chamber in Exeter, was treated for suspected cerebral embolism (air bubbles in the brain resulting from damage to the lungs). The nurse, suffering from severe earache, had to be decompressed again to relieve the pain and remove any air bubbles that might have formed.

Hyperbaric oxygen therapy is not available to Britain's estimated 85,000 MS sufferers on the NHS, but the Federation of Multiple Sclerosis Therapy Centres, a charitable organisation with more than 70 chambers in its centres around the country, including the one in Exeter, offers the treatment free.

The use of oxygen chambers at these centres is not directly regulated by the Department of Health, local health authorities, or the Health and Safety Executive, but the federation says that it has its own strict protocols for using them and its voluntary staff are highly qualified.

The Exeter centre treats about 90 people a week in its chamber and was recently awarded pounds 300,000 of National Lottery money. Sue Earle, its medical liaison officer, says that in this incident there was no question of malpractice. The federation, which has a system of self- inspection, is, however, carrying out an inquiry.

"We have voluntarily invited the Health and Safety Executive in to inspect us in the past and they were extremely complimentary about our running protocols," said Ms Earle. "We have a highly qualified chamber operator with more than 30 years' experience. Everyone who goes into the chamber is fully briefed about the procedures and asked about their medical background."

She added that the centre would welcome regulation by the local health authority.

Although it is common practice for nurses and relatives to accompany severely disabled patients into the chambers, to help them with oxygen masks, many neurologists say this puts them at risk.

Dr David Bates, a consultant neurologist at the Royal Victoria Infirmary in Newcastle, says: "The mechanics of asking young, healthy people to accompany patients into chambers is a problem," he says.

Dr Bates, author of a major study of hyperbaric oxygen therapy, adds: "I know of no neurologist who would advise this treatment. Poor MS sufferers tend to have everything thrown at them, but this simply doesn't work."